1. Field of the Invention
The present invention relates to a polishing method, for example, of optical crystals, a method of manufacturing an optical device and a liquid suspension used for polishing.
2. Description of Related Art
In recent years, as the wavelength of optical sources used for semiconductor lithographic or the like has become shorter, optical crystals (for example, non-linear optical device, electrooptic device and acousto-optic device) have been used in short wavelength regions such as UV-wavelength regions.
In such optical crystals, surface scattering of light incident to an optical surface is given by the following relation: EQU S=(4.pi..delta./.lambda.).sup.2 (1)
where S is scattering, .delta. is root mean square for surface roughness and .lambda. is light wavelength (refer to J. M. Bennet and L. Mattson, "Introduction to Surface Roughness and Scattering" pp. 25). That is, as the light wavelength become shorter, the effect of the surface roughness on the optical surface to the surface scattering increases.
Accordingly, in order to use optical crystals in a short wavelength region (for example, UV-wavelength region), it is necessary to attain smaller surface roughness compared with an optical device used in a region of a relatively long wavelength such as a visible region or an IR region. In particular, the scattering loss results in a significant problem upon wavelength conversion of a laser beam by utilizing a non-linear optical effect (for example, generation of second harmonics or third harmonics).
In the optical mirror polishing technique of the prior art for optical crystals used for lasers, a liquid suspension prepared by dispersing a fine powder, for example, of CeO.sub.2 or SiO.sub.2 as a polishing material into water or an aqueous solution such as an aqueous alkali solution has been used as a liquid suspension for polishing in a polishing step (refer to G. W. Fynn and W. J. Powell, "Cutting and Polishing Optical and Electronic Materials", pp. 88).
However, if crystals themselves have water solubility or deliquescence such as .beta.-BaB.sub.2 O.sub.4 crystals (BBO crystals) developed in recent years, for example, for UV-wavelength conversion, since the crystals cause deliquescence in the presence of a water soluble solvent in this method, polishing is impossible in principle (refer to D. Eimerl, et al., J. Appl. Phys., 62 (5), pp. 1968, 19879).
As a method of polishing a material with poor water resistance, there have been known, for example, a method of using a liquid suspension for polishing in which a fine alumina powder is dispersed in ethylene glycol (refer to G. W. Fynn and W. J. Powell, "Cutting and Polishing Optical and Electronic Materials", pp. 201), and a method of using a liquid suspension for polishing in which diamond is dispersed in glycerine (refer to Japanese Patent Unexamined Publication Hei 2-1333999).
However, the methods described above often result in micro-scratches to the surface of the material to be polished (optical crystals). Such micro-scratches, as fine defects on the optical surface (hereinafter sometimes referred to as optical defects), would lower the destruction threshold of optical crystals to incident light, can not increase the optical density of the incident light in application and, further, give hindrance to the application uses for high optical density and high power lasers. Further, the micro-scratches form latent flaws in the steps with heating such as vapor deposition or annealing after mirror polishing, which developed on the optical surface of the device to give undesired effects on the device characteristics (refer to Adachi "Optical Device Fabrication Technology, 1989", 1-2 "Optical Material and Fabrication", pp. 63.